1. Technical Field
The present disclosure relates to a resonant biaxial accelerometer structure of a MEMS (microelectromechanical system) type, in particular capable of detecting with high electrical performance two independent components of acceleration in a plane.
2. Description of the Related Art
As is known, MEMS accelerometers are currently used, thanks to their extremely compact dimensions, low levels of consumption, and good electrical performance, in a wide range of fields of application, amongst which include the automotive industry, monitoring of vibrations, and portable electronics.
The numerous MEMS accelerometers proposed in the literature and currently present on the market can be generally grouped into three classes, on the basis of the principle of detection used by the corresponding microelectromechanical detection structure: capacitive, resonant, and piezoresistive.
In resonant accelerometers, the external acceleration to be measured produces a detectable shift of the resonance frequency of the microelectromechanical structure, or of some part thereof. The resonant detection, as compared to other measurement principles, presents the advantage of offering a direct frequency output, high sensitivity, and wide dynamic range.
In greater detail, the external acceleration is detected in terms of a shift in the resonance frequency of a resonant element, in general beam-shaped, coupled to an inertial mass (the so-called “proof mass” or “free mass”).
An external linear acceleration a produces a force F on the inertial mass m, with F=m·a; said force in turn produces an axial action N, proportional thereto (and hence proportional to said external acceleration a) in the resonant element, which is appropriately kept in a resonance condition by an electronic circuit coupled thereto. The axial action hence determines a variation of the natural resonance frequency, designated by f, of the resonant element, according to the following relation:
                    f        =                              f            0                    ·                                    1              +                              α                ⁢                                                                  ⁢                                                      NL                    2                                    EI                                                                                        (        1        )            where f0 is the fundamental frequency of the resonant element without axial load, given by:
                              f          0                =                                            c              2                                      2              ⁢              π              ⁢                                                          ⁢                              L                2                                              ·                                    EI                              ρ                ⁢                                                                  ⁢                A                                                                        (        2        )            
and moreover L, A and I are, respectively, the length, the area of the cross section, and the moment of inertia of the resonant element, ρ is the mass density of the material of which it is made, E is the elastic modulus, and c and α are coefficients, the value of which depends, in a known way, upon the conditions of constraint of the beam that constitutes said resonant element.
If the external acceleration is angular, instead of linear, a torque is generated, proportional to the polar moment J of the mass, which induces, in a way similar to what has been discussed previously, an axial action on the resonant element, varying the frequency thereof according to the aforesaid relation (1).
Various accelerometers based upon the resonant operating principle have been proposed and fabricated by the semiconductor technologies, and in particular by means of techniques of “bulk micromachining” or, more recently, of “surface micromachining”. With reference to resonant accelerometers obtained with surface-micromachining techniques, the following documents may for example be cited:    M. Aikele, K. Bauer, W. Ficker, F. Neubauer, U. Prechtel, J. Schalk, H. Seidel “Resonant accelerometer with self-test”, Sensors and Actuators A, 92, 161-167, 2001;    A. A. Seshia, M. Palaniapan, T. A. Roessig, R. T. Howe, R. W. Gooch, T. R. Shimert, S. Montague “A vacuum packaged surface micromachined resonant accelerometer”, JMEMS, 11,784-793, 2002;    L. He, Y.-P. Xu, A. Qiu “Folded silicon resonant accelerometer with temperature compensation”, Sensors 2004. Proceedings of IEEE, 1, 512-515, 24-27 Oct. 2004;    S. X. P. Su, H. S. Yang, A. M. Agogino “A resonant accelerometer with two-stage microleverage mechanisms fabricated by SOI-MEMS technology” Sensors, 5(6), 1214-1223, 2005.
The various MEMS resonant accelerometers so far proposed differ from the standpoint of the geometries envisaged for the microelectromechanical detection structure (in particular for the different arrangements of the resonant element with respect to the inertial mass), and consequently for the electrical characteristics that derive therefrom, for example as regards the amplification of the axial force and consequently the sensitivity in the detection of acceleration. In particular, the sensitivity of resonant accelerometers is generally defined as the variation in frequency generated by an external acceleration the equal to 1 g.
Known resonant accelerometers obtained through techniques of surface micromachining typically have a sensitivity that starts from some tens of Hz/g and does not exceed 200 Hz/g, and, at least some of them, have rather large dimensions.
In addition, the MEMS resonant accelerometers proposed so far are for the most part of a uniaxial type, i.e., able to detect, with a single inertial detection mass, a single component of acceleration directed along a given axis of detection. Consequently, it is necessary to replicate the microelectromechanical structures proposed, each provided with a corresponding inertial mass, to obtain a detection of components of acceleration directed along a number of axes of detection.
The present applicant has recently proposed in U.S. patent publication application 2011/0056294, a microelectromechanical structure for a resonant accelerometer, of a uniaxial type, which has a high sensitivity and reduced dimensions, thanks to the particular geometrical arrangement of the constitutive elements. Also the teachings of said patent application do not regard, however, a structure with a number of axes of detection that is compact and has high electrical performance.